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Abstract
An all-fiber seawater salinity sensor based on intracavity loss-modulated sensing in a fiber ring laser is proposed and experimentally demonstrated. An optical fiber multimode interferometer, which is based on single-mode-no-core-single-mode fiber structure, is cascaded with a fiber reflector and used as a reflected sensing head to enhance loss-modulated depth. It is inserted in a fiber ring laser and the intracavity loss-modulated salinity sensing is induced for the fiber laser’s output intensity. The salinity sensitivity is measured to be 0.1 W/‰ with a high signal-to-noise ratio more than 49 dB and narrow full width at half maximum less than 40 pm. The temperature cross-sensitivity characteristic and stability are also analyzed. Considering the errors from cross-sensitivity, stability and resolution of the photodetector, the detection limit of the sensor system is 0.0023 ‰ (0.0002 S/m), which is comparable to the most advanced commercial electronic salinity sensor.
© 2019 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Measurement of seawater salinity plays an indispensable role in a variety of scientific applications, including aquaculture, monitoring and management of marine environment, oceanography research, mineral exploration, etc. Conventional commercial salinity sensors based on electronic components collect salinity information by probing the conductivity of seawater. Normally, these measurements are carried out in conjunction with depth and temperature by means of a so-called CTD (conductivity, temperature, depth) device. In recent years, as a potentially attractive alternative, optical fiber salinity sensors have been received widespread attention. Salinity information can be obtained by measuring the refractive index (RI) of seawater [1]. Existing forms of optical fiber salinity sensor include conventional long period grating based interferometer [2], microfiber knot-type ring resonator [3,4], in-fiber Fabry-Perot cavity [5], side-polished two-core fiber [6], U-shaped single mode fiber [7], and various tapered specialty fibers [8] and microfiber optical fibers [9–11]. In order to improve the sensitivity of optical fiber salinity sensors, most of them are fabricated by fused biconical tapering or laser drilling. Most of these sensors have a high sensitivity with the detection limit from 0.05‰ to 0.005 ‰. However, they are hard to be constructed. Moreover, the sensor head of these sensors either has poor robustness or enlarges the temperature cross-sensitivity of the sensing fiber. The fragile sensing structure become unreliable and unsuitable for measurement in extreme marine environment. Recently, optical fiber sensors based on fiber laser intracavity modulation have been demonstrated for magnetic field and temperature measurement [12,13]. These sensors are featured with good sensitivity, high signal-to-noise ratio (SNR) and narrow full width at half maximum (FWHM). They have a great potential in marine detection.
In this paper, an optical fiber sensor for salinity measurement based on fiber laser intracavity modulation and reflection enhancement technology is demonstrated. As a loss-modulated device, an optical fiber multimode interferometer (MMI), constructed by single-mode-no-core-single-mode fiber (SNCS) structure, is inserted in the fiber laser ring cavity to modify the operation loss of the ring cavity. A fiber reflector is cascaded with the SNCS structure to enhance modulation sensitivity. By the two technologies mentioned above, the detection limit (DL) is up to 0.0023 ‰ (0.0002 S/m), which is comparable to the most advanced commercial electronic salinity sensor. The seawater salinity information is obtained by seawater RI through the conversion relationship in [1]. The double-sleeve structure is designed for protection in seawater, which ensures the dependability of sensing part exposed to seawater. And the sealing structure is capable of withstanding high water pressure and preventing infiltration into the instrument.
2. Experimental setup and principle
The experimental setup of the proposed optical fiber salinity sensor is schematically shown in Fig. 1. In a fiber ring cavity, a section of 3-meter-long Erbium-doped fiber (EDF, Nurfen, EDFC-980-HP) serves as the gain medium, which is pumped by a 976-nm diode laser (AFR Inc.) via a wavelength division multiplexing coupler (WDM, AFR Inc., 980/1550 nm). An isolator (ISO) is employed to maintain the unidirectional light propagation and prevent spatial hole-burning [14]. The SNCS structure constructed by no-core fiber (NCF) and two standard single-mode fibers is cascaded with a fiber reflector (Hefei Wave Photonics Co., Ltd) and used as a reflective sensing head, which is shown in Fig. 2(b). The sensing head is inserted in the fiber ring cavity by a circulator and performs as an edge filter. A fiber Bragg grating (FBG) is linked in the same way and works as a wavelength selective filter. So, the lasing wavelength of the fiber ring laser is at the central wavelength of the FBG. The picture of package structure is shown in Fig. 2(a). The sealing structure is capable of withstanding high water pressure. Furthermore, the double-sleeve structure is designed for antifouling, among which strainer mesh and UV LEDs (Ultra-Violet Light Emitting Diodes) are used for preventing the sensing fiber from adhesion of marine biomolecules and plankton. The reflective sensing head is fixed on the detective protection structure by epoxy glue as depicted in Fig. 2(c). Figure 2(d) shows a completed sensing probe. The output spectrum is measured by an optical spectrum analyzer (OSA, YOKOGAWA, AQ6370, spectral resolution 0.02 nm) via a 10:90 coupler.
Fig. 2 (a) The package structure of the sensing head, (b) The structure of the SNCS fiber, (c) The SNCS fiber fixed on the protective structure, (d) a completed sensing probe.
The sensing principle of SNCS structure is multimode interference [15]. Assuming that the single-mode fibers and NCF are ideally aligned, only the symmetric modes, i.e. the {LP0m} modes, will be excited when the fundamental mode in the input fiber enters the NCF and then the interference between these modes will occur. Neglecting the attenuation caused by the surrounding medium at the NCF section, the transmittance at the wavelength λ can be calculated by [16]
where is the total number of the excited modes in the NCF, and are the excitation coefficient and propagation constant of the {LP0i} mode, respectively, and L is the length of the NCF.According to Eq. (1), the transmission loss of SNCS structure will be minimized if the phase changes of all the modes along the NCF differ by integer multiples of 2π when these modes are coupled back to the output single-mode fiber. The phenomenon is known as self-imaging [15], which means that an input field profile is reproduced at the splice between the NCF and the output fiber. The corresponding peak wavelength can be expressed as [17]
where is an integer, and respectively represent the effective refractive index and diameter of the fundamental mode of the NCF. The characteristics of high modulated depth and low insertion loss contribute to an ideal edge filter. Due to splicing loss and ineluctable center offset, the transmission loss of self-imaging peak is around −3 dB in practice.In our sensor system, the resonant wavelength of the FBG is designed at ~1532 nm, overlap to the gain peak position of EDF, to obtain a high SNR. Its reflective spectrum is shown in Fig. 3(a). The reflective sensing probe is constructed by the SNCS structure cascaded with a fiber reflector and it is easily assembled for engineering equipment. Furthermore, the reflective device is proposed to enhance the intensity-modulated depth owing to the light modulated twice at the SNCS structure. As the surrounding salinity of the sensing head rises due to the increase of seawater RI, the spectrum of the SNCS has a red shift [18] and the operation loss of the fiber ring cavity will be modulated. In order to obtain a monotonous response characteristic, wavelength shift need to be estimated according to the RI range of seawater. The RI of seawater ranges from 1.3382 to1.3408 [19]. The calculated sensitivity of RI response characteristic of SNCS structure is around 118 nm/RIU [18] in the relevant range. In the experiment, the SNCS structure is fabricated by a section of 6.0-cm NCF (Tianjin Opticaland Co., Ltd, diameter ~125 μm) spliced two standard single-mode fibers. Due to the short self-imaging length, the length of the NCF is difficult to exactly control in practice. So, several samples were fabricated and the device with suitable transmission peak was selected to use in this demonstration. Its peak wavelength is at 1525.13 nm in pure water. The salinity response characteristic of the refractive sensing head is described in Fig. 3(a). As the surrounding seawater salinity increases from pure water to 165.2385 ‰ at 20 °C, the insertion loss of the sensing head declines from −12.92 dB to −10.57 dB at the position of the resonant wavelength of the FBG. Figure 3(b) shows the output power variations of the fiber laser in different surrounding salinities at different pump power. The insert in Fig. 3(b) is the output power of the sensor system as pump power increases when the sensing head is in pure water. The threshold of the sensor system is about 50mW. The result shows that the pump power has a little impact on the output power variations in different surrounding salinities.
Fig. 3 (a) The reflection spectrum of the FBG and the salinity response characteristic of the sensing head, (b) The salinity responses of the output power at different pump power, the insert is the output power as pump power increases when the sensing head is in pure water.
3. Results and discussion
3.1 Salinity sensing
To stabilize the output power of the sensing system, the sensing head is fixed on the inner wall of a square flume filling with solution. The salinity changing of seawater can be simulated by solution of different concentrations. The RI of solution can be calibrated by Abbe refractometer (WAY-2W, Shanghai Precision Scientific Instruments Co., Ltd.) and then the salinity can be calculated. The output spectra of the fiber laser are measured in Fig. 4(a) as the surrounding salinity of the sensing head increases from pure water to 160.3195 ‰. It shows that the output power rises as the salinity increases. The salinity response of the output power of the fiber ring laser monitored by a power meter (OMM-6810B, ILX Lightwave, resolution 1 pW) is linearly fitted in Fig. 4(b). The salinity sensitivity is measured to be 0.1 μW/‰, which has a great linear response with R2 of 0.994. As is known to us, the output power of fiber laser is extremely sensitive to intracavity loss, which can be modulated by the SNCS structure in our system. The SNCS structure device decreases the intracavity loss of the fiber laser as the surrounding salinity increases. So, the output power of the sensor system has a high sensitivity to surrounding salinity. Compared with wavelength and phase demodulation, intensity demodulation has a higher sensing precision. The sensor system has a narrow FWHM less than 40 pm due to the narrow filtering characteristic of the FBG. The sensor system has a Q-factor of 104 calculated by . The SNR and relative intensity sensitivity of the fiber laser sensor are measured as shown in Fig. 4(c). The insets are the output spectra of the fiber laser measured in pure water and 160.3195 ‰, respectively. As the surrounding salinity increases from pure water to 160.3195 ‰, the SNR of the sensor goes up from 49 dB to 53 dB, which has a sensitivity of 0.02088 dB/‰, correspondingly. The sensing range of the salinity sensor can be adjusted by changing the length of NCF because the wavelength of self-imaging peak is decided by it. The narrow FWHM and high SNR make the sensor has a great potential in high-capacity sensor network and remote detections.
Fig. 4 (a) The output spectra of the fiber laser as the external salinity increases from pure water to 160.3195 ‰, (b) The salinity response of the output power, and (c) The SNR and relative intensity sensitivity of the sensor as the external salinity increases from pure water to 160.3195 ‰, the insets are the output spectra of the fiber laser measured in pure water and 160.3195 ‰, respectively.
Output stability is another important parameter for intensity-modulated sensors which limits their accuracy of measurement. In order to observe the stability of the laser sensor, the output spectra of the fiber laser sensor are monitored over 200 minutes by fixing the surrounding salinity at pure water, 65.7666 ‰, 128.0731 ‰ and 160.3195 ‰, respectively. The stability is measured when the sensor system works at room temperature. The results are shown in Fig. 5. The power fluctuations are monitored by the power meter four hours a day for five days. The average values of the output power per hour are recorded, which is described in Fig. 6. The fluctuations at different salinity make the output to be 29.83 ± 0.11 μW at pure water, 37.59 ± 0.06 μW at 65.7666 ‰, 43.19 ± 0.10 μW at 128.0731 ‰, and 47.03 ± 0.13μW at 160.3195 ‰. The corresponding measurement error induced by power fluctuation is no more than 0.34% when the mean-value power is used for salinity sensing. In the measurement of power fluctuation, the relevant standard deviation is no more than 75.57 pW (σstab-included). The proposed sensor system shows a good intensity stability and small measurement errors.
Fig. 5 The output spectra of the fiber laser sensor monitored over 200 minutes by fixing the external salinity at (a) pure water, (b) 65.7666 ‰, and (c) 128.0731 ‰, (d) 160.3195 ‰, respectively.
3.2 Cross-sensitivity analysis of temperature
The output power variation of the fiber ring laser is induced by the spectral shift of the SNCS structure response. To our knowledge, the spectrum of SNCS structure also has a red shift due to the increase of the environmental temperature [18]. So, the cross-sensitivity of temperature has been analyzed.
The output power of the fiber laser is monitored as the surrounding temperature rises from 10 °C to 50 °C in pure water. It is shown in Fig. 7 that the output power of the fiber laser increases slowly with temperature increasing. The temperature sensitivity is 0.037 μW/°C. The commercial temperature sensor in CTDs can be used for temperature compensation. Assuming that the accuracy of the equipped temperature sensor is 0.001 °C, which is close to the temperature precision of most CTDs. The calculated error caused by temperature compensation can be around 0.037 nW and the standard deviation of σtemp-included = 10.68 pW.
3.3 Summary of sensor performance parameters
For a detector with a power resolution of 1 pW, the error in determining output power of the sensing system is uniformly distributed between −0.5 pW and 0.5 pW. The calculated standard deviation is 0.2887 pW (σpower-res). Once the statistical properties of all of the errors are understood, the sensor resolution can be determined. Here we use the typical convention of establishing the resolution as 3σ of the total error in the system. The total system error can be approximated by summing all of the individual errors, i.e. . And the sensitivity (S) and the sensor resolution (R) combine to form the DL of the sensor system [20]:
In our sensing system, the DL reports the smallest salinity change of seawater that can accurately be measured. All of the sensor performance parameters are listed in the Table 1. The detection limit of our salinity sensor is equivalent to 0.0023 ‰ (0.0002 S/m), which is comparable to the precision of the most advanced commercial electronic salinity sensor. Among all the errors, output stability is the main factor that affect the precision of the sensor. The accuracy of the proposed salinity sensor will be further improved if output stabilization is under more effective control.
Table 1. Salinity Sensor Indicators
4. Conclusion
In conclusion, we demonstrated a high-accuracy all-fiber salinity sensor. High-sensitivity salinity measurement is realized by the sensor system combining an optical fiber MMI based on SNCS structure with the intracavity intensity modulation in a fiber ring laser. Reflection enhancement structure is applied to improve the sensitivity of the sensor system by making light pass the SNCS structure twice. The salinity sensitivity is measured to be 0.1 μW/‰. Correspondingly, relative intensity sensitivity achieves 0.02088 dB/‰. The sensor has a high SNR more than 49 dB and narrow FWHM less than 40 pm. The temperature cross-sensitivity characteristic and stability have been also analyzed. Considering the errors from cross-sensitivity, stability and resolution of the photodetector, the detection limit of the sensor system is 0.0023 ‰ (0.0002 S/m), which is comparable to the most advanced commercial electronic salinity sensor.
Funding
National Basic Research Program of China (973) (2015CB755400, 2015CB755403); National Key Research and Development Programs (2016YFC0101001, 2017YFA0700202); National Natural Science Foundation of China (NSFC) (81802118, 61735010, 61775160, 61771332, 61675147).
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